Fabregas is worldclass. Yes, he was not a key member cause Xavi is better. Wich isnt saying much, since Xavi is spains greatest midfielder in history. Actually, in Barcelona's playstyle, what midfielder during Xavi's prime was superior to him ? I cant think of anyone.
Your dortmund argument is bad. Forwards always gather interest quickly. It doesnt mean they are more important. Gundogan is their best midfielder, and arguably tied with aubameyang for best player at this point. Reus is below that.
Talking about interest. Yeah, aubameyang is hot. Klopp is wanting gundogan at liverpool. Van gaal wants him at manchester. Reus ? nothing much. Interest will increase as the season progresses, because players who excell but dont score or assist, draw attention much slower.
I said, Isco is "top 3 primadonna am". You can look for my post to find it. He plays in the team with the biggest competition for his position. He is not a starter, but he plays alot of games, on several positions. he's not happy, because he wants to be the starter, the key player. He wont get that at madrid.
However. Alot want him to stay at madrid. Chelsea and manchester city want him.
Liverpool, Man Utd, Man City and Chelsea want two of those players. The other one has impossible wages and will find a team that pays them long before he considers to cut his wages to come here.
Sure, we can afford any of them if we wouldnt have competition. But as you can see, we quite have.